1. Field of the Invention
The present invention relates to communication systems, and more particularly to a communication system in which a transmitting station and a receiving station are communicably connected through a transmission path, and the receiving station reserves a bandwidth, which is used in data communication, for the transmitting station in advance.
2. Description of the Background Art
Conventionally, a multiple access method has been applied to some communication systems. As an example thereof, “Splid-channel Reservation Multiple Access” (IEEE Trans. Commun., vol. COM-24, pp. 832 to 845, August 1976) is described next below by referring to FIG. 16. The communication system shown in FIG. 16 includes a central station 121, which controls bandwidths for communication, and secondary stations 122 and 123. Hereinafter, a description is made for a case where the secondary stations 122 and 123 transmit data to the central station 121.
For data transmission, the secondary stations 122 and 123 first transmit request packets 124 and 125 to the central station 121 using a reserved channel. After receiving the request packets 124 and 125, the central station 121 schedules timing for data packet transmission so as to reserve bandwidths for the secondary stations 122 and 123. And then, the central station 121 creates a response packet 126 for transmission to a channel for response.
The secondary stations 122 and 123 respectively receive and analyze the response packet 126 to find which bandwidths are each assigned thereto. Through the assigned bandwidths, the secondary stations 122 and 123 respectively transmit data packets 127 and 128 created in accordance with data.
In such SRMA (Splid-channel Reservation Multiple Access), it is necessary to reserve the bandwidth for every data packet in advance to transfer the same. In other words, data packet transmission always follows bandwidth reservation.
For bandwidth reservation, the request packets 124 and 125 and the response packet 126 are transmitted and received. The time requisite for transmission and reception thereof results in heavy overhead and bandwidth usage to a considerable degree.
Further, the stations 122 and 123 are allowed to freely send out the request packets 124 and 125 to any channel for reservation. Accordingly, the request packets 124 and 125 may come into communication collision with each other if being transmitted with the same timing.
If this is the case, the central station 121 cannot correctly receive the collided request packets 124 and 125, and thus cannot create the response packet 126 for transmission. Therefor, the stations 122 and 123 are required to transmit the once-transmitted request packets again, thereby rendering the data communication delayed.